Our distant ancestors may have swung from branches and knuckle-walked like a chimpanzee – challenging recent thinking that the earliest hominins did neither. That is the conclusion of an analysis of 4.4-million-year-old Ardipithecus ramidus, thought to be one of the earliest known hominins.
In popular thinking, humans are often imagined to have evolved from a chimpanzee-like ape, but many researchers now challenge this idea – particularly in light of fossil evidence from A. ramidus that was published in 2009. One well-preserved individual – nicknamed Ardi – had bones that suggested it typically walked along branches like a monkey rather than swinging below them like a chimp. This hinted that our last common ancestor with chimps also walked along branches, and that chimps evolved to swing and knuckle-walk after they branched off from hominins.
Thomas C. Prang at Texas A&M University and his colleagues disagree with this conclusion. They have taken the measurements of Ardi’s hands reported in 2009 and compared them with 416 measurements from hands across 53 species of living primates, including chimpanzees, bonobos and humans.
“The analysis of this hand, one of the earliest hands in the human fossil record, suggests that it is chimpanzee-like, implying that both humans and chimps evolved from an ancestor that was chimp-like,” says Prang.
They found that Ardi’s metacarpals and phalanges – the bones of the fingers and palms – were similar in size to those of living apes, with relatively large joint and knuckle dimensions. These adaptations are present in existing primates that move around forests by swinging below branches and may have helped the hominin to grasp onto branches, and even knuckle-walk.
“Ardi also has elongated, more curved finger bones, and we see this increased elongation and curvature in animals that habitually hang from branches,” says Prang.
Larger-bodied primates tend to hang from branches and climb trees, while smaller-bodied animals, like monkeys, are able to walk along the branches.
“[The study] quite convincingly demonstrates that the Ardipithecus hand has some suspensory adaptations, which I think makes more sense given the body size,” says Tracy Kivell at the University of Kent, UK.
The researchers also confirmed this using evolutionary modelling. This involved comparing traits of different primates, both living and extinct, to understand the evolutionary relationship between physical features and movement. “In short, this approach models the evolution of traits across a tree of life, which in this case includes all the species in our analysis,” says Prang.
Understanding hand morphology of our earliest human relative brings us one step closer to explaining why humans are so different from our close relatives. This may suggest that the last common ancestor of chimpanzees and humans was relatively chimpanzee-like, before the major evolutionary shift towards bipedalism and hand dexterity.
Tim White at the University of California, Berkeley, who discovered the A. ramidus fossil and helped describe it in 2009, remains unconvinced. “This is another failed resurrection of the antiquated notion that living chimpanzees are good models for our ancestors,” says White. He says that the Ardipithecus hand, aside from having five fingers and the ability to grasp, wasn’t specifically chimpanzee-like, as he and his colleagues originally reported in 2009.
Sergio Almécija at the American Museum of Natural History in New York is also largely unconvinced. “We need more Miocene [epoch] ape fossils pre-dating the human-chimp split to test fundamental aspects of our last ancestor with apes,” he says.
Journal reference: Science Advances, DOI: 10.1126/sciadv.abf2474
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